As the nuclear industry is making progress recently, various nuclear facilities, such as reactors and fuel reprocessing plants, are built in many places. In these nuclear facilities, there is a need to render the irradiation dose of a human body as low as possible and prevent the structural materials and materials for equipment from suffering from damage due to irradiation. Specifically, neutrons generated from fuel or spent fuel (or recycle fuel) in the nuclear facilities have high energy and high penetrating power, and collide with a substance to generate γ-rays, causing the materials in the nuclear facilities to suffer from the damage. Therefore, a neutron shield which can surely shield neutrons and γ-rays with safety is being developed.
Concrete has been conventionally used as a neutron shield, however, when the concrete is used as a shielding wall, it needs a considerable thickness and hence is unsuitable for neutron shield in a nuclear facility having a limited capacity, such as a nuclear ship, and therefore a neutron shield having a reduced thickness has been desired.
When a neutron, especially fast neutron collides with a hydrogen element having substantially the same mass, the hydrogen absorbs energy of the neutron, achieving effective neutron moderation. Therefore, a material having a high hydrogen density, namely, high hydrogen content is effective in shielding fast neutrons, and, for example, water, paraffin, or polyethylene can be used as a neutron shield. A liquid such as water is lightweight, as compared to concrete, but the liquid has a limited form of handling, and further a material for a container itself for containing a liquid, such as water, must be considered in respect of the neutron shielding ability.
Therefore, for shielding neutrons, there are used materials obtained by incorporating a slight amount of a boron compound as a neutron shielding material into a resin having such a high hydrogen content that it remarkably effectively serves as a neutron moderator, for example, a polyolefin thermoplastic resin, such as paraffin or polyethylene, a thermosetting resin, such as an unsaturated polyester resin, or an epoxy resin. On the other hand, for shielding γ-rays, a structure for shielding γ-rays having a form such that it surrounds the neutron shield body is provided.
Further, the development of a neutron shield which can maintain the neutron shielding ability at a certain level or more even when a fire occurs is made. With respect to this, a neutron shield having incorporated thereinto a large amount of aluminum hydroxide powder or magnesium hydroxide powder as a refractory material has been proposed (Japanese Patent Application Laid-open No. 2001-108787, Japanese Patent Publication No. 3150672).
However, in the shielding neutrons, a neutron shield which can more surely shield neutrons with safety is desired. On the other hand, in the shielding γ-rays, the neutron shield comprised of a resin, which is mainly used currently, has a specific gravity as small as 0.9 to 1.2 and hence is not suitable for shielding γ-rays generated when shielding fast neutrons. Therefore, there has been a need to provide a structure for shielding γ-rays using a material having a larger specific gravity around the neutron shield body. In other words, the current neutron shield cannot satisfy itself both the neutron shielding ability and the γ-ray shielding ability.
The fact that there is a need to provide a structure for shielding γ-rays using a material having a larger specific gravity around the neutron shield body is not the best mode in a nuclear facility having a limited capacity, and the improvement of the neutron shield not only in the neutron shielding ability but also in the γ-ray shielding ability is expected.
With respect to the fire resistance, the dehydration heat decomposition temperature of aluminum hydroxide is 245° C. to 320° C., whereas, the dehydration heat decomposition temperature of magnesium hydroxide is 340° C. to 390° C., and therefore it is considered that magnesium hydroxide powder is more suitable as a refractory material. However, when magnesium hydroxide powder is used, there are a problem that the viscosity of the resultant composition increases and kneading and filling of the composition require a prolonged time and burdensome operations, and a problem that voids caught in the resin are likely to remain to lower the neutron shielding ability. For this reason, there has been no example of actual use of magnesium hydroxide powder in the cask, and no example of studies on the particle size of magnesium hydroxide powder, either.
The present invention has been achieved with a view to the above problems, and an object of the invention is to provide a composition for neutron shield, which is advantageous not only in that it effectively shields both neutrons and γ-rays, but also in that it has excellent fire resistance, and a cask using the same. It is also an object of the invention to provide a composition for neutron shield having excellent workability.